It is suggested that sedimentary deposition of iron in sediments from the Archean to the present day can be attributed largely to microbial mediation. Results from laboratory experiments, using a microbial consortium enriched from the source of a biofilm growing on a rock face in an underground research laboratory, are used to advance a plausible explanation for the mobilization, precipitation, transport, and deposition of iron. The consortium produces its own local environments independent of the prevailing atmosphere. It is able to extract iron from minerals such as biotite and magnetite, as well as from a chelated solution; this iron is then metabolized, mainly through dissimulatory iron reduction, to provide cell energy, after which it is immediately precipitated. All organisms require energy for growth and reproduction, but because iron redox reactions are inefficient a large amount of iron must be processed, either directly through metabolism or indirectly due to the local microbial redox microenvironment, with the result that vast quantities accumulate as waste. This precipitate could be the main source of iron in sedimentary iron formations. Siderite and ferrihydrite, the main precipitates, may occur in close juxtaposition within a biofilm. The oxidation state of the iron precipitate is controlled by the nutrient supply, which in turn influences the metabolism of the biofilm organisms and hence their redox. Subsequently, this iron, enmeshed within the biofilm, is either deposited in sediments as fine layers or rolled by wave or current action into particles and granules, which can form structures similar to those found in banded iron formations (BIFs).